High strength aluminum alloy

ABSTRACT

High strength aluminum alloys and methods for producing them. The alloys consist essentially of about 9.0 to 10.3 wt. % zinc, about 2.5 to 3.5 wt. % magnesium, about 1.5 to 3.0 wt. % copper and less than about 0.05 wt. % of any other alloying constituent. The balance consists of aluminum. These alloys are compatible with ceramic reinforcements used in metal matrix composites.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of aluminum alloys and,more particularly, to alloys produced by powder metallurgy processes.Such alloys are compatible with ceramic reinforcements used in metalmatrix composites.

2. Background Art

The development of a high strength aluminum alloys based on theAl—Zn—Cu—Mg alloy system for many years has focused on three mainapproaches: (1) heat treatment variation to maximize propertycombinations, (2) thermal-mechanical treatments and (3) second phasechemistry control.

SUMMARY OF THE INVENTION

The present invention provides aluminum alloy products having improvedstrength and fatigue resistance. The present invention further providesa method of producing improved aluminum alloys by powder metallurgyprocesses. These alloys are compatible with ceramic reinforcements usedin metal matrix composites. These alloys are characterized by high yieldstrength and elastic modulus at room temperature and are thereforeuseful in aircraft and other demanding applications.

In one embodiment, an aluminum alloy consists essentially of about 9.0to 10.3 wt. % zinc, about 2.5 to 3.5 wt. % magnesium, about 1.5 to 3.0wt. % copper and less than about 0.5 wt. % of any other alloyingconstituent. The balance consists of aluminum, although it will beunderstood that there may also be trace amounts of unavoidableincidental impurities. This new alloy has at least about 10% greateryield strength than 7050-T6 aluminum, one of the strongest aluminumalloys currently in wide use for demanding aerospace applications.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, for purposes of explanation and notlimitation, specific numbers, dimensions, materials, etc. are set forthin order to provide a thorough understanding of the present invention.However, it will be apparent to one skilled in the art that the presentinvention may be practiced in other embodiments that depart from thesespecific details. In other instances, detailed descriptions ofwell-known speaker components are omitted so as not to obscure thedescription of the present invention with unnecessary detail.

Aluminum alloys in accordance with embodiments of the present inventionare made by powder metallurgy processes, such as vacuum-hot-pressing orcold-isostatic pressing and sintering. The alloys can be made byblending elemental powders with aluminum powders to create the desiredalloy. The alloys can also be made by blending aluminum powder withmaster alloys containing the desired alloy ingredients. The alloypowders can also be made by atomizing a melt with the desiredcomposition. In particular embodiments, the alloy contains aluminum withbetween 9.0 and 10.3 percent zinc, 2.5 and 3.5 percent Mg, 1.5 and 2.2percent copper and less than about 0.5 percent (preferably less thanabout 0.05 percent) of any other alloying constituent. This alloy can beused as the matrix for a particle reinforced composite. The alloy ismade with fine powders in order to control the grain size andmicrostructure of the final product. The maximum particle size for thepowder is 44 microns.

The fine powders for this alloy are blended in commercial units that arecompatible with fine aluminum alloys. A ceramic powder that will act asa reinforcement can be added to the alloy powder and blended at thistime. Ceramic materials suitable for use as the reinforcement phaseinclude silicon carbide, silicon nitride, SiAlON, titanium nitride,titanium carbide, titanium silicide, molybdenum silicide, nickelaluminate, boron carbide, aluminum nitride, aluminum oxide, magnesiumoxide, silicon and mixtures thereof. The powders may be isostaticallycompressed into a cohesive or coherent shape. This can be effected byplacing the powders within a bag, such as a rubber or plastic material,which in turn is placed within a hydraulic media for transmittingpressure through the bag to the powder. Pressures are then applied inthe range of 5 to 60 psi which compress the powder into a cohesive shapeof about 85 to 93% of full density. This isostatic compaction stepfacilitates handling of the powder. The isostatically compacted materialcan then be sintered by placing the compact in a vacuum furnace andheating to temperatures of 875° F., preferably 900° or 950° F., whilecontinuing to pull a vacuum down to a pressure level of one torr,preferably 10⁻¹ or 10⁻² torr or less (1 torr=1 mm Hg at 0° C.). Thedensity of the sintered billets remain between 90 and 95 percent of thetheoretical and must be metal worked by extrusion, forging or rolling inorder to develop full density and full properties.

Alternatively, the material can be compacted to substantially fulldensity at relatively high temperatures. This can be effected by placingthe powder or compacted material in a hard tool that is placed in acontainer and evacuating the container at room temperature and heatingto temperatures of 675° F., preferably 700° or 850° to 950° F., whilecontinuing to pull a vacuum down to a pressure level of one torr,preferably 10⁻¹ or 10⁻² torr or less (1 torr=1 mm Hg at 0° C.). Whilestill in the sealed container, the material is compressed tosubstantially full density at temperatures of 900° to 950° F.

When referring to substantially full density, it is intended that thecompacted billet be substantially free of porosity with a density equalto 95% or more of the theoretical solid density, preferably 98 or 99% ormore. It is desired that the vacuum compaction to full density beeffected at a minimum temperature greater than 650° F., for instance675° F. or higher, and preferably at a minimum temperature of 700° F. orhigher. The maximum temperature for compaction should not exceed 960° F.After being compacted to substantially full density at elevatedtemperature and vacuum conditions as just described the billet which canthen be shaped such as by forging, rolling, extruding or the like or canbe machined into a useful shape. It is preferred that the billet beworked by any amount equivalent to a reduction in cross section of atleast 25%, preferably 50 or 60% or more, where practical, since suchfavors improved elongation properties. Preferred working temperaturesrange from 550° to 850° F.

After working the product, it is heat treated to the desired conditionand quenched. The product is then aged within a temperature range ofabout 235° F. to 270° F. for about 6 to 60 hours.

Several materials were made with fine powders with different chemicalcontents. The zinc content was varied from 8.4 to 11 percent, themagnesium content was varied between 2 and 2.9 percent and the coppercontent was varied between 1.25 and 2 percent. The alloy billets wereextruded into 0.625 inch diameter rods from a 3.5 inch container. Therods were cut into sample blanks. The sample blanks were heat treated toa T-6 condition by solution treating at 900° F. for 1 hour, roomtemperature water quenched and then aged for 24 hours at 250° F. Roomtemperature tensile tests were conducted on specimens machined intoreduced section bars and the results are presented in Table 1. The yieldstrength goal for the new alloy was set at 20% higher than 7050, or 85ksi. The data indicate that all of the alloys have yield strengthsgreater than 85.

TABLE 1 Strength Data for High Strength Aluminum Alloys Ulti- StrainMaterial Ceramic Yield mate at Descrip- Content Strength StrengthFailure tion Mg Zn Cu (v %) (ksi) (ksi) (%) 7050* 1.6- 5.7- 2.0- — 71 809 2.6 6.7 2.6 A 2.7 9.6 2 0 102.7 104 7.7 B 2.7 9.6 2 1 101.4 105 7.3 C2.7 9.6 2 3 100 103 5 D 2.7 9.6 2 5 95 100 5 E 2.9 10.00 1.50 10 97.8100.6 2.42 F 2.2 11.00 1.25 10 90.0 95.3 3.24 G 2 8.40 1.70 5 92.0 98.08.50 H 2.7 9.00 1.50 10 95.3 100.2 3.01 I 2.5 9.00 2.00 10 85.2 93.62.30 J 2.2 9.49 1.50 10 91.0 101.9 2.68 K 2.2 9.50 1.50 5 90.7 95.8 5.03L 2.7 9.50 2.00 10 104.5 109.0 3.03 M 2.7 9.50 2.00 10 100.0 104.8 2.25N 2.7 9.50 2.00 10 85.2 93.6 2.30 O 2.9 9.50 2.00 10 98.3 100.5 2.30 P2.5 9.50 2.00 10 94.0 97.2 2.89 Q 2.9 9.50 2.00 10 95.8 100.4 2.98 *Thedata for 7050 is from the Mil Handbook 7-G.

The chemical composition limits for alloys in accordance withembodiments of this invention are defined in Table 2.

TABLE 2 Alloy Element Composition Limits Min Max Si 0.15 Fe 0.2 Cu 1.53.0 Mn — Mg 2.5 3.5 Cr — Ni — Zn 9.0 10.3 Ti Ga — V — Zr — Oxygen  0.050.50 Others, Each 0.05 Others, Total 0.15

It will be recognized that the above-described invention may be embodiedin other specific forms without departing from the spirit or essentialcharacteristics of the disclosure. Thus, it is understood that theinvention is not to be limited by the foregoing illustrative details,but rather is to be defined by the appended claims.

1. An aluminum alloy consisting essentially of about 9.0 to 10.3 wt. %zinc, about 2.5 to 3.5 wt. % magnesium, about 1.5 to 3.0 wt. % copperand less than about 0.5 wt. % of any other alloying constituent, thebalance aluminum.
 2. A metal matrix composite comprising a metal phaseconsisting essentially of about 9.0 to 10.3 wt. % zinc, about 2.5 to 3.5wt. % magnesium, about 1.5 to 3.0 wt. % copper and less than about 0.5wt. % of any other alloying constituent, the balance aluminum, and areinforcement phase selected from the group consisting of siliconcarbide, silicon nitride, SiAlON, titanium nitride, titanium carbide,titanium silicide, molybdenum silicide, nickel aluminate, boron carbide,aluminum nitride, aluminum oxide, magnesium oxide, silicon and mixturesthereof.
 3. The metal matrix composite of claim 2 wherein the metalphase comprises about 50 to 99 vol. %.
 4. The metal matrix composite ofclaim 2 wherein the reinforcement phase is selected from the groupconsisting of particulates, whiskers, fibers and mixtures thereof.
 5. Amethod for producing a high strength aluminum alloy product comprising:(a) providing an aluminum-base alloy consisting essentially of about 9.0to 10.3 wt. % zinc, about 2.5 to 3.5 wt. % magnesium, about 1.5 to 3.0wt. % copper and less than about 0.5 wt. % of any other alloyingconstituent, the balance aluminum; (b) working the alloy into a product;(c) heat treating the product; (d) quenching the heat treated product;and (e) aging the product within a temperature range of about 235° F. to270° F. for at least 6 hours.
 6. A method for producing a high strengthaluminum alloy matrix composite product comprising: (a) providing analuminum-base metal phase consisting essentially of about 9.0 to 10.3wt. % zinc, about 2.5 to 3.5 wt. % magnesium, about 1.5 to 3.0 wt. %copper, and not more than about 0.5 wt. % of any other alloyingconstituent, the balance aluminum; (b) blending said metal phase with atleast one reinforcement phase consisting essentially of a gradation ofparticles sizes having 90% less than minus 325 mesh; (c) working saidblended metal phase and reinforcement phase to produce a product; (d)heat treating said product; (e) quenching said product; and (f) agingsaid product within a temperature range of about 235° F. to 270° F. forat least 6 hours.
 7. The method of claim 6 in which said reinforcementphase is selected from the group of particulates, whiskers, fibers andmixtures thereof.
 8. The method of claim 6 in which said reinforcementphase is selected from the group consisting of silicon carbide, siliconnitride, SiAlON, titanium nitride, titanium carbide, titanium silicide,molybdenum silicide, nickel aluminate, boron carbide, aluminum nitride,aluminum oxide, magnesium oxide, silicon and mixtures thereof.
 9. Themethod of claim 6 in which said blended metal phase and reinforcementphase comprises 50 to 99 vol. % of said metal powder phase and 1 to 50vol. % of said reinforcement phase.
 10. The method of claim 6 in whichsaid metal phase comprises a pre-alloyed powder.
 11. The method of claim6 in which said metal phase comprises a mixture containing at least oneelemental powder.